$-8st + 5t + 2u - 1 = 8t + 9u - 9$ Solve for $s$.
Combine constant terms on the right. $-8st + 5t + 2u - {1} = 8t + 9u - {9}$ $-8st + 5t + 2u = 8t + 9u - {8}$ Combine $u$ terms on the right. $-8st + 5t + {2u} = 8t + {9u} - 8$ $-8st + 5t = 8t + {7u} - 8$ Combine $t$ terms on the right. $-8st + {5t} = {8t} + 7u - 8$ $-8st = {3t} + 7u - 8$ Isolate $s$ $-{8}s{t} = 3t + 7u - 8$ $s = \dfrac{ 3t + 7u - 8 }{ -{8t} }$ Swap the signs so the denominator isn't negative. $s = \dfrac{ -{3}t - {7}u + {8} }{ {8t} }$